Reviewing a drafted flood risk assessment properly

Reviewing a flood risk assessment is mostly a matter of checking that the document answers the question it was asked, with data that exists and applies to this site. Most weak assessments are not wrong about hydraulics. They are wrong about scope: the wrong flood zone source, a climate change allowance lifted from a different river basin, finished floor levels quoted without a datum, or an emergency plan that assumes somebody will approve it who never does.

Work through it in the order the reader will, and check the evidence behind each statement rather than the statement itself.

Checking the flood zone and the source of the data

Start with where the zone came from. The national Flood Map for Planning shows Flood Zones 2 and 3 and ignores the presence of defences, which is the point of it. A Flood Risk Assessment that reports a zone without naming the source and the date it was viewed has already lost a mark.

Flood Zone 3b, the functional floodplain, is the one to look for. It is not shown on the national map. It is defined by the local planning authority in its strategic flood risk assessment, and it is where the most restrictive policy applies. An assessment that says the site is in Zone 3 without distinguishing 3a from 3b has skipped the question that matters.

Where the site is near a zone boundary, a postcode-level check is not enough. Look for a site-specific enquiry to the Environment Agency for modelled levels, and check whether the assessment used them or estimated. Estimated levels dressed as modelled outputs are the most serious fault you can find in one of these documents.

Whether all sources of flooding are addressed

Fluvial and tidal risk is the easy part. The sources that get skipped are surface water, groundwater, sewers, and artificial sources such as reservoirs, canals and ponds.

Check each of the following is addressed, with a source given:

• Surface water risk from the national long term flood risk service, at property scale rather than a screenshot of a wider area.
• Groundwater, using borehole and geological information and any records of historic flooding.
• Sewer flooding, which means an enquiry to the sewerage undertaker rather than an assumption.
• Reservoir inundation mapping where a reservoir sits upstream.
• Historic flooding records held by the lead local flood authority.

A report that dismisses a source in one sentence without citing anything has not assessed it. The correct form is to name the dataset, state what it shows and then explain why the risk is or is not material.

Climate change allowances and the epoch used

This is where technical errors concentrate. The Environment Agency publishes peak river flow allowances by river basin district and management catchment, with different allowance categories and different epochs, and separate allowances for peak rainfall intensity and sea level rise.

Four checks. Is the allowance taken from the correct river basin district and management catchment for this site. Is the epoch correct for the development's lifetime, which is normally taken as 100 years for residential and shorter for commercial. Is the allowance category appropriate to the flood zone and the vulnerability classification of the use. And is the version of the published allowances current, with the date of access stated.

An allowance copied from a report on a different river is the single most common technical error in assessments produced at volume, and it is easy to spot because the catchment name will not match the site.

Where the assessment relies on an automated draft, look for the markers. PlanScope's drafts insert a [TO BE CONFIRMED] flag where a site-specific input is missing rather than supplying a plausible figure, and a reviewer's first job is to check that every one of those flags has been closed with real data rather than deleted.

Finished floor levels and freeboard

Levels have to be expressed against Ordnance Datum and supported by a topographic survey. A level given as a height above existing ground is not usable, because existing ground varies across the plot.

The common expectation is a finished floor level set above the modelled 1 in 100 annual probability level including climate change, with a freeboard allowance on top, often 300 mm. That figure is not universal and the Environment Agency or the lead local flood authority may ask for more, so check whether the assessment states the requirement it is meeting and where that requirement comes from.

Then check the consequences have been followed through. Raising floor levels changes access thresholds, affects level access under Building Regulations, changes the relationship with neighbouring properties, and may change the drainage design. An assessment that sets a level and stops has not finished the job, and the Drainage Strategy has to be consistent with it.

Safe access and egress claims

This section is asserted far more often than it is demonstrated.

A safe access claim needs the route shown on a plan, with levels along it, and a statement of the depth and velocity of flooding on that route during the design event. Hazard is a function of depth and velocity together, and published guidance on flood hazard rating gives the method. A route under 200 mm of slow-moving water is a different proposition from the same depth moving quickly.

Dry access is the preferred outcome. Where it cannot be achieved, the assessment should say so plainly and explain what is proposed instead, whether that is safe refuge above the flood level, a flood warning and evacuation plan, or a restriction on the use.

Be sceptical of evacuation plans. The Environment Agency generally does not approve them, and emergency planners are not a consenting authority. A plan that depends on a third party accepting a role that third party has not agreed to is not mitigation.

Sequential and exception test evidence

The sequential test is applied by the local planning authority, but the applicant supplies the evidence. Check that the area of search was agreed with the authority, that the sources of alternative sites are named, that the assessment of each is recorded rather than summarised, and that the conclusion follows from the analysis. A Sequential Test that searches a two-mile radius with no explanation of why that radius was chosen will not hold.

Where the exception test applies, both parts have to be addressed separately: the wider sustainability benefits, and whether the development will be safe for its lifetime without increasing flood risk elsewhere. The wording of the test has changed between editions of national policy, so check which edition the assessment is working to and confirm it against the current published Framework. An Exception Test that recites the older formulation is a sign the document was assembled from an out-of-date template.

Residual risk and the emergency plan

Residual risk means what happens when the design assumptions fail: a breach, an event larger than the design event, a blocked culvert, a pump failure.

A competent residual risk section identifies the mechanism, states the consequence, and sets out what reduces it. Flood resilient construction below the design level, raised electrical services, non-return valves on drainage, and flood warning registration are the usual measures. Each should be tied to a condition or it will not happen.

Finally, check the document is proportionate. A householder extension in Flood Zone 2 does not need a 60 page report, and padding one out is as much a failure of judgement as omitting a source.

Flood risk assessments are documents where a suitably qualified author is expected, and professional review before submission is strongly advised. Reviewers can be found through the professional directory, and PlanScope pricing covers drafting the supporting documents around the technical work.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.